Synthesis methods and electrochemical performances for TiO2-based materials.
With the increased attention on sustainable energy, a novel interest has been generated towards construction of energy storage materials and energy conversion devices at minimum environmental impact. Apart from the various potential applications of titanium dioxide (TiO2), a variety of TiO2 nanostructure (nanoparticles, nanorods, nanoneedles, nanowires, and nanotubes) are being studied as a promising materials in durable active battery materials. The specific features such as high safety, low cost, thermal and chemical stability, and moderate capacity of TiO2 nanomaterial made itself as a most interesting candidate for fulfilling the current demand and understanding the related challenges towards the preparation of effective energy storage system. Many more synthetic approaches have been adapted to design different nanostructures for improving the electronic conductivity of TiO2 by combining with other materials such as carbonaceous materials, conducting polymers, metal oxides etc. The combination can be done through incorporating and doping methods to synthesize TiO2-based anodic materials having more open channels and active sites for lithium and/or sodium ion transportation. The present chapter contained a broad literature and discussion on the synthetic approaches for TiO2-based anodic materials for enhancing the lithium ion batteries (LIBs) and sodium ion batteries (SIBs) performance. Based on lithium storage mechanism and role of anodic material, we could conclude on future exploitation development of titania and titania based materials as energy storage materials.
- Titanium Dioxide
- Lithium Ion Battery
Nowadays, investigation in the field of energy storage and conversion devices with different functionalities is emerging subject for many research investigators [1, 2, 3]. Since last two decades, the regular efforts are being implemented towards the development of nanostructured as electrochemical storage materials convenient to access both surface and bulk properties and hence a superior storage and conversion performance. In this view, nanostructured materials are of great interest due to easy availability for modification into degree of crystallinity, phase, particle size, morphology and porosity which are prior characteristics.
As a talk of nanomaterials and nanotechnology, titania (TiO2) and titania-based materials always been studied first. TiO2 nanoparticles (NPs) are being widely investigated over the past few decades due to their unique characteristics such as non-toxicity, abundance, thermal and chemical stability, and easy availability. Many more research and progress reports on TiO2 have shown a great potential in various important applications such as photocatalysis, biomedical, environmental remediation and many more. Beyond these applications, TiO2 and TiO2-based nanomaterials also offers novel materials to overcome the energy and environment related challenges. Different TiO2 nanostructures with high surface area, large pore volumes, tunable pore structures and nano-confined effects have been synthesized and used in various fields with excellent performances . In the past, many comprehensive reviews have been documented on synthesis, properties and applications of TiO2 and TiO2-based nanomaterials.
In view of energy storage technologies, recently, lithium-ion batteries (LIBs) are found to be emerging technologies for imperative electric grid applications such as mobile electronics, electric vehicles and renewable energy systems operating on alternating energy sources like wind, tidal, solar and other clean energy sources [5, 6]. The performance of these technologies in terms of capacity, recyclability and rate capability are much more dependent on the characteristics of the active anode and cathode materials. The performance can be improved through fundamental modification with particular strategy with such factors like the power capacity, long term durability and most prior its cost. In this view of direction, finding of energy storage materials with high efficiency and low cost is thrust challenge for the materials scientists. As we talked about various important characteristics of TiO2, it could be suitable candidate due to its versatile functionalities. The present chapter covering literature on the recent progress of applications of TiO2 and TiO2 based materials as energy storage technologies and discussion on the efforts that have been made so far.
Theoretically, the anode part has a crucial role in LIBs and thus, the direction towards development of anode materials is one of the most essential factors which could define the performance of the device . As an ideal anode material, it should possess high specific surface area allowing better insertion for mobile ions (lithium ions for LIBs); large pore size, low volume change and low internal resistance for speedy charging and discharging; low intercalation potential for mobile ions; and operating at moderate condition with economical and environmental benefits.
Among available various suitable anode materials, transition metal oxides in which TiO2 is following the characteristics of an ideal anode material that makes TiO2 itself as most promising anode material for LIBs. Apart from these benefits and utilities of TiO2, some drawbacks still exist like low capacity and poor rate capability [4, 7]. Thus, TiO2 suffering from poor ionic/electronic conductivity that limits the lithium storage rate. However, the transport of electrons and Li+ ions can be promoted by engineering of their physicochemical and morphological characteristics as presented in Figure 1. In this view, many more researches and efforts have been made to overcome the said disadvantages by designing and adopting different synthetic strategies to obtain various forms of TiO2 such as zero-dimensional (0D) nanospheres, one dimensional (1D) nanostructures, two-dimensional (2D) nanoarchitectures and three-dimensional (3D) hierarchical nanostructures with different electronic structures. These nanostructures are showing the advantages of providing high contact surface area with the electrolyte as well as short diffusion pathways for electrons and mobile ions such as Li+ and Na+. In addition, the adoption like doping of different heteroatoms into TiO2 lattice which could alter the chemical and physical surface of TiO2 would open more channels and active sites for transportation of mobile ions due to which electrical conductivity can be increased .
2. Synthesis approaches and nanostructural forms
Generally, synthetic strategy is crucial tool to design the structural characteristics due to which materials functionality can be optimized for better performance. Interestingly, TiO2 can be synthesized in such manner to have different structural forms and hence, versatile functionalities. To summarize recent structural development of titania, this chapter started with a brief introduction on the synthesis and structural characteristics of TiO2 which has great impact on the performances.
In search of synthesis routes for fabrication of TiO2-based materials, sol–gel, hydrothermal/solvothermal and impregnation methods of preparation are well recognized. Sol–gel synthesis is mode of preparation offering optimization of various experimental parameters at moderate conditions and allows designing material structure with customized properties. Generally, sol–gel route is widely used to synthesize hybrid and composite types materials by using aqueous and non-aqueous mode in suitable solvent media. In this finding, performance based composite-TiO2 materials to be used as energy storage materials have been synthesized by sol–gel route. Hydrothermal process consist of treatment of bulk TiO2 with alkaline solution at high temperature and pressure using Teflon reactor or autoclave instrument. The reaction conditions of pressure and temperature yields selective product which decide the morphology and functionalities of the materials. By use of these useful synthetic techniques, various kind of TiO2-based materials can be prepared as anode materials in LIBs or/and SIBs applications.
2.1 Carbon coating approach
This approach has received much attention providing superior electrical conductivity, large surface-to-volume ratio, and excellent mechanical and chemical stabilities. In this view many more research works have been done successfully. Kim
MWCNT array has been grown on a SiO2 precursor via combustion chemical vapor deposition (CCVD) method and then prepared TiO2 precursor have been deposited onto MWCNT/SiO2. The characterization results for the obtained material confirmed positive preliminary data about the capacitance and energy density. The analyzed data proved to be potential as an improvement over comparable electrochemical capacitance device. The present idea of hybridization can be used in current electrochemical theory to create an energy storage device that is both efficient and inexpensive . He
In addition, graphene oxide (GO) can be considered as another conducting carbon source which has a good ability to form a composite with TiO2 by using various synthetic techniques. Many more significant research works are in progress to find feasibility for GO through combined with TiO2 in the field of energy storage materials. Farooq
2.2 Doping approach
This has been well established method for the introduction of impurities into the semiconductor crystal to intentionally change its conductivity due to deficiency or excess of electrons . Tanaka
2.3 By using conducting polymer
This approach also has gained much attention because of high electrical conductivities and good redox properties of such conducting polymeric materials. Polyoxometalates (POMs), a well-known class of transition metal oxide nanoclusters with interesting structures and diverse properties can give a nanocomposite of TiO2. In this view, Qu
2.4 Combination with other semiconductors
2.5 Impregnation method
Hierarchical TiO2 sphere composed of ultrathin nanotubes consist of both anatase and rutile phases have been synthesized from a low-temperature hydrothermal reaction and calcination process . The obtained hierarchical TiO2 (~78% anatase and ~ 21% rutile phases) used as-prepared electrode mixed with acetylene black and binder of sodium-CMC in a weight ratio of 7:2:1 using distilled water as solvent. The solution was placed on a copper foil as the current collector and the electrode dried in a vacuum oven at 100°C for 24 h. From this electrochemical tests, prepared material showing the superior performance in Li-storage with a specific capacity of ~167 mA·h·g−1 at 1 A·g−1 current of 6C and maintained ~187 mA·h·g−1at rate of 10 A·g−1 at current rate of 60C. The capacity retention rate was ~97% at 1 A g and ~ 92% at 2 A·g−1 after 500 cycles. The present performance is attributed to the presence of both phases of TiO2 due to which the synergistic effect has been originated between the defective anatase and rutile renders the shared conduction of electrons through the anatase and Li+ ions via the rutile at high current rates. In addition, pathway for electron and Li+ ion conduction was found to be shortened. Also, the interfacial boundaries between the two phases can contribute to extra capacitance at high rates by forming a Li+/e− double layer. Overall, the synergy between the anatase and rutile phases, and interfacial storage are revealed to be beneficial for the high-kinetic reaction in lithium-ion batteries . Anatase phase mesoporous TiO2 has been synthesized using the urea assisted hydrothermal process and used for high power performance for LIBs applications . The material obtained as mono-phasic TiO2 sub-microspheres of uniform particle size with I41/amd space group. From the rate capability behavior, material has a specific charge capacity of ~162 mA·h·g−1 at current rate of 0.5 C and slightly reduced to 160, 154 and 147 mA·h·g−1 at 1, 5 and 10C-rates, respectively. The obtained good performance due to the large surface area (~116 m2·g−1) introduced by the highly porous (pore size of ~7 nm) nano-structured building blocks of each anatase TiO2 sub-microspheres resulting to make favorable and short diffusion pathway for ionic and electronic diffusion. The near zero strain behavior of this anatase phase mesoporous TiO2 makes it a suitable anode material for high power lithium applications.
TiO2 bulk NPs have been synthesized using modified hydrothermal process under aqueous medium . Their behavior as an intercalation host for Li+ ions has been explored by performing an electrochemical test in which the active material (TiO2) mixed with poly(vinylidene fluoride) and Super P carbon in the weight ratio of 70:20:10, then introduced into an electrochemical cell along with a lithium metal counter/reference electrode and liquid electrolyte. The mixture was cast onto copper foil from acetone using the Doctor-Blade technique. From this test, gravimetric capacity for the TiO2 bulk NPs at all rates up to 18000 mA·g−1 was found to be identical to the 6 nm anatase particles which have a much higher proportion of carbon content. In comparison from nanowires to nanotubes and nanoparticles, the amount of Li and hence charge that can be stored, even at low rates, increased with reduced dimensions. The volumetric capacity of composite electrodes with nanoparticulate TiO2 bulk was found to be notably high than other titanate materials at rated above 1000 mA·g−1.
Table 1 summarizes discussed synthetic modification processes for TiO2-based materials to be used as anode materials for LIBs or SIBs.
|Sr. No.||TiO2 based anode materials and method of synthesis||LIBs or SIBs performance||Ref.|
|1||Anatase TiO2 nanorods synthesized by a hydrothermal method followed by carbon coating||Capacity of ~193 mA·h·g−1 on the first charge in a sodium cell|||
|2||Porous anatase TiO2 synthesized by aqueous sol–gel process followed by coatings on carbon nanotubes (CNTs)||Reversible capacity as high as ~200 mA·h·g−1 at a current density of 0.1 A·g−1 for LIBs|||
|3||CNT@mesoporous TiO2 hybrid nanocables prepared by a combined sol–gel and hydrothermal route by using hexadecylamine as a structure directing agent||Discharge capacity as high as ~183 mA·h·g−1 at current of 1 C for LIBs|||
|4||TiO2/MWCNT composite prepared using via a combined sol–gel and solvothermal method||Discharge capacity as high as ~316 mA·h·g−1 with good reversibility and stability after 100 cycles|||
|5||Hybrid mesoporous CNT@TiO2-C nanocable using anatase TiO2, CNT and glucose (as carbon source) as for structure directing agent.||charge capacity of ~187 mA·h·g−1 after 2000 cycles at current of 5 C and ~ 122 mA·h·g−1 after 2000 cycles at current of 50 C for LIBs|||
|6||Prepared TiO2 precursor deposited onto MWCNT/SiO2 using combustion chemical vapor deposition (CCVD) process||Performance that is significantly better and more applicable than current electrochemical capacitors for LIBs|||
|7||Hierarchical rod-in-tube structure TiO2 with a uniform conductive carbon layer using solvothermal method||Discharge capacity values of ~277 mA·h−1·g−1 at 50 mA·g−1 and ~ 153 at 5000 mA·g−1 and retained over 14000 cycles|||
|8||Anatase TiO2 NPs anchored on CNTs using ammonia water assisted hydrolysis and ||Cycling stability values of ~92 mA·h·g−1 remained at a current density of 10 A·g−1 at current of 60 C in LIBs|||
|9||Phase-pure anatase TiO2 nanofibers with fiber-in-tube and filled structures by electro-spinning process using tetra-||Li-ion storage capacity (~231 mA·h·g−1)|||
|10||Self-supported, three-dimensional single-crystalline nanowire array electrodes by using simple hydrothermal process: TiO2, TiO2-C and TiO2-C/SnO2 have been produced on flexible Ti foil||Discharge capacity higher than ~160 mA·h·g−1 and a capacity retention rate of ~84% even after 100 cycles at a current rate of 10 C|||
|11||A single crystalline TiO2, TiO2-C and TiO2-C/MnO2 core-double-shell nanowire arrays on flexible Ti foil through a layer-by-layer deposition technique||Performance of ~332 mA·h·g−1 at current of 2 C in LIBs|||
|12||TiO2 NPs on reduced GO nanosheets by a microwave hydrothermal process||Discharge capacity of ca. 100 mA·h·g−1 with >99% coulombic efficiency at C-rates of up to 20 C in LIBs|||
|13||Hydrothermal process to synthesize 3D mesoporous TiO2 nanocubes grown on RGO nanosheets without use of any surfactants and high temperature of calcination||LIBs with high specific capacity value of ~180 mA·h·g−1 at current of 1.2 C after 300 cycles|||
|14||Mixed detonation nanodiamond (DND) and TiO2 hollow nanospheres synthesized by sol–gel route||LIBs with reversible capacity value of ~348 mA·h·g−1 at current of 0.5 C after 100 cycles|||
|15||Nb-doping mesoporous TiO2 by one-pot solvothermal process using carboxylic acids as organic additives in supercritical methanol medium.||Li-extraction capacity of ~147 mA·h·g−1 at the 1000th cycle at a high current rate of 10 C|
Na-extraction capacity of ~128 mA·h·g−1 upto 1000 cycles
|16||TiO2 ceramics co-doped with niobium (Nb5+) and erbium (Er3+)||Better dielectric properties than the singly-doped ones|||
|17||Hybrid bifunctional nanocomposite film based on TiO2 nanowires and Polyoxometalates (POMs) using combination of hydrothermal and layer-by-layer self assembly methods||Coloration efficiency (~150 cm2·C−1 at 600 |||
|18||Nanocrystalline TiO2 mixed with V2O5 with different compositions using sol–gel synthesis process at different calcination temperatures||Energy storage ability: Initial charging rate of ~3700 C·mol−1·h−1|||
|19||Form-stable composite of 1-tetradecanoic acid (TDA) and TiO2 powder using a facile impregnation method||Superior thermal stability with latent heat storage capacity of ~97 J·g−1|||
|20||Hierarchical TiO2 sphere composed of ultrathin nanotubes having mixed anatase and rutile phases synthesized from a low-temperature hydrothermal reaction and calcination||Specific capacity of ~167 mA·h·g−1 at 1 A·g−1 current of 6 C and maintained ~187 mA·h·g−1 at 10 A·g−1 at current of 60 C|||
|21||Anatase phase mesoporous TiO2 synthesized using the urea assisted hydrothermal process||Specific charge capacity of ~162 mA·h·g−1 at current rate of 0.5 C and slightly reduced to 160, 154 and 147 mA·h·g−1 at 1, 5 and 10 C-rates, respectively|||
|22||Porous TiO2 samples have been synthesized by simple thermal oxidation of titanium foil||LIBs Stable for 200 cycles|||
|23||Nanocrystalline anatase TiO2 electrodes synthesized by using sodium carboxymethyl cellulose (CMC) binder, and in combination with styrene butadiene rubber (SBR) via aqueous process||Energy density of ~ >120 mW·h·g−1|||
|24||TiO2 bulk NPs synthesized using modified hydrothermal process under aqueous medium||High capacity than other titanate materials at rated above 1000 mA·g−1.|||
3. Role of anode in LIBs: the aging mechanism
The anodic materials is operating in organic electrolyte such as LiPF6 with co-solvents like ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC) [33, 34]. Generally, the most important part of the LIBs cell is the anode/electrolyte interface because of high reactivity of the organic electrolyte with any type of electrode material and lithium ions for LIBs. Due to the interaction occurring between the composite anode and the electrolyte solution would form organic species on the anode surface that can be recognized as solid electrolyte inter-phase (SEI). The produced organic species can undergo reduction with CO2 and traces of H2O in the electrolyte to form lithium carbonate which further react with used organic solvents to form transesterification products. On other hand, anion contaminates, such as F− from HF and PF5, readily react with lithium to form insoluble reaction products which are non-uniform, electronically insulating, and unstable on the surface of the anodic material. At higher battery potentials, during the intercalation of lithium ions into the anode lattice structure, within the reactions continue, the consumption of lithium ions with co-solvents can precipitate and grow on the anode surface. This process consequently results in the formation of a protective, ionically conductive but electrically insulating passive layer on the surface of the anode during the first charge cycle, the so-called SEI. This SEI creates resistance to lithium ion flow, which results in a rise in the charge transfer resistance and the impedance of the anode. This increase follows charge rate, cycle number, temperature, and surface area/particle size of anodic material. The stable growth of SEI may leads to the loss of active lithium and further decomposition of the electrolyte. This phenomenon is the main degradation mechanism in fully charged batteries at storage conditions [33, 34].
Furthermore, interactions of the anode with the cathode should be included because of the dissolution of the cathode electrode metal from the lattice into the electrolyte solution may occur when batteries stored at voltages greater than ~3.6 V . This may cause deposition of cation contaminates which are incorporated into the SEI layer. This kind of reactions can damage SEI and a short circuit may be generated, which then can lead to thermal runway and battery failure.
Fracture and decrepitating of the electrodes are critical challenges existing in lithium-ion batteries as a result of lithium diffusion during the charging and discharging operations. A large volume change on the order of a few to several hundred percent can be observed due to lithium ions intercalate and de-intercalate. Thus, diffusion-induced stresses (DISs) generate the nucleation and growth of cracks/damages, leading to mechanical degradation of the active electrode materials. Therefore, for nanoscale electrode structures, surface energies and surface stresses can be predictable to have an important impact on the mechanical properties of the electrode materials.
3.1 Effect of particle size, active surface area and porosity of the anode material
Generally, small particles contain short diffusion paths between the anodic particles, which make easy and fast charge and discharge rate the. Similarly, large surface area of the anodic material are prone to higher internal heat generation and lithium ion are consumed during the exothermic reaction at high temperatures (> 60°C) compared to larger particles size, this leads to an enhancement in the irreversible capacity of the anodic material. There is no a direct connection between the porosity of the anode and the reversible capacity of the anode. At high temperature (> 120°C), heat generation from a denser electrode material produces gaseous species through thermal decomposition of the SEI layer. This reveals about the importance of the thermal stability of an anodic material. Overall, the anode of the lithium ion battery undergoes several degradation mechanisms during aging. Ion (Li+) plating is one aging mechanism which ends the life of a battery more rapidly due to the formation and growth of lithium dendrites. These kinds of degradation mechanisms rarely affect the crystal structure of the anode electrode.
In comparison with Li-ion insertion mechanism where Li3OCl forms as an intermediate, the presence of Na3OCl is not observed as an intermediate product after the first conversion due to it is metastable and its limited lifetime .
In conclusions, the huge amounts of efforts are in progress towards the synthesis, characterization and application of TiO2 based anode materials in the fields of LIBs. Development can be seen with an aim to gain the superior electrochemical performance and promote the practical utilities of the synthesized TiO2-based functionalized materials. The synthesis strategies have played a crucial role to establish a material composition with a desired nanostructure and appropriate morphologies including particle size/shape and surface area by adopting different modification methods as discussed in the chapter. The effective synthesis condition can easily provides material informations with unique characteristics and hence high lithium or sodium storage capacity and cyclic stability which are depend on ion flux at the electrode/electrolyte interface, internal resistance and diffusion path.
The energy storage capacity strongly influenced by materials structure and morphologies, thus various structural forms should be explored to enhance the electrochemical performance of modified TiO2 materials. The chapter providing a bunch of literature reports on how synthetic process can alter the nanostructure that facilitates the electrochemical performance at minimum cost and good durability. The formation of one dimensional (1D), two dimensional (2D) and three dimensional (3D) hierarchical nanostructures have been found to be much stable materials for electrochemical performance. The doping approaches also can open more windows to modified TiO2 matrices and therefore different structural forms. The uniformly carbon coating also making a huge impact on formation of TiO2 based modified materials with good surface area and electronic conductivity favoring good energy storage performances. The use of various carbon sources like CNTs, MWCNT and graphene oxide have formed the homogenously dispersed nanocomposites showing a stable cyclic performance for LIBs.
Overall, progressive research works have been well established for TiO2 to be used as anode materials in the field of energy storage. Although, still challenges are there to improve the Li ion storage performance like low coulombic efficiency, low volumetric energy density etc. To solve the fundamental issues, more development towards material surface alteration or coating to reduce unwanted side reactions and designing hierarchical structural materials by adopting different experimental conditions.
In addition, on other hand, sodium ion battery (SIBs) getting more attention which has to be used as thrust area of research. The prepared materials should be performed for LIBs and SIBs as well.
The authors thank the University Grants Commission (UGC), New Delhi, India, for financial assistance under the Dr. D.S. Kothari Post Doctoral Fellowship (No. F.4.-2/2006/(BSR)/CH/16-17/0049).
Conflict of interest
The authors declare no conflicts of interest.
Acronyms and abbreviations
Lithium Ion Batteries
Sodium Ion Batteries
Reduced Graphene Oxide
Multi-walled Carbon Nanotubes
Combustion Chemical Vapor Deposition
Electrochemical Impedance Spectroscopy
Styrene Butadiene Rubber
Methyl Ethyl Carbonate
Solid Electrolyte Inter-phase
Appendices and nomenclature for scientific units